Selective Nitridation Crafted a High‐Density, Carbon‐Free Heterostructure Host with Built‐In Electric Field for Enhanced Energy Density Li–S Batteries

Abstract To achieve both high gravimetric and volumetric energy densities of lithium–sulfur (Li–S) batteries, it is essential yet challenging to develop low‐porosity dense electrodes along with diminishment of the electrolyte and other lightweight inactive components. Herein, a compact TiO2@VN heterostructure with high true density (5.01 g cm–3) is proposed crafted by ingenious selective nitridation, serving as carbon‐free dual‐capable hosts for both sulfur and lithium. As a heavy S host, the interface‐engineered heterostructure integrates adsorptive TiO2 with high conductive VN and concurrently yields a built‐in electric field for charge‐redistribution at the TiO2/VN interfaces with enlarged active locations for trapping‐migration‐conversion of polysulfides. Thus‐fabricated TiO2@VN–S composite harnessing high tap‐density favors constructing dense cathodes (≈1.7 g cm–3) with low porosity (<30 vol%), exhibiting dual‐boosted cathode‐level peak volumetric‐/gravimetric‐energy‐densities nearly 1700 Wh L−1 cathode/1000 Wh kg−1 cathode at sulfur loading of 4.2 mg cm−2 and prominent areal capacity (6.7 mAh cm−2) at 7.6 mg cm−2 with reduced electrolyte (<10 µL mg−1 sulfur). Particular lithiophilicity of the TiO2@VN is demonstrated as Li host to uniformly tune Li nucleation with restrained dendrite growth, consequently bestowing the assembled full‐cell with high electrode‐level volumetric/gravimetric‐energy‐density beyond 950 Wh L−1 cathode+anode/560 Wh kg−1 cathode+anode at 3.6 mg cm−2 sulfur loading alongside limited lithium excess (≈50%).

annealing at 600 °C for 2 h in NH 3 atmosphere, the resultant TiO 2 @VN were obtained. For comparison, the two control samples, TiO 2 and VN, were also prepared separately by the same process as the preparation of TiO 2 @VN.

Synthesis of the TiO 2 @VN-S, TiO 2 -S, and VN-S composites
To prepare TiO 2 @VN-S composites, the as-prepared TiO 2 @VN and sulfur were firstly mixed uniformly by the ratio of 3:7. Then, the mixed powder was transferred into the tubular furnace and heated at 155 °C for 12 h under Ar atmosphere. Before cooling down, the as-prepared sample was annealed at 250 °C for 2 h to remove the redundant sulfur on the external surface of the TiO 2 @VN. Finally, the TiO 2 @VN-S composites were obtained. For comparison, the TiO 2 -S and VN-S composites were obtained in the same way.

Electrochemical measurements of S-composite cathode
The TiO 2 @VN-S cathode was fabricated by mixing 70% TiO 2 @VN-S composites with 20% Super P and 10% PVDF in NMP to form homogeneous slurry and then coated on the Al/C foil dried at 60 °C for 12 h, shaped into circular plates with 12 mm. Then, the cathode was assembled with Li foil anode and Celgard 2400 separator into CR2032 coin cells inside glove box with H 2 O and O 2 below 0.1 ppm. For the pouch cell assembly, the TiO 2 @VN-S cathode and lithium anode were cut into 33 cm pieces. The sulfur loading of the cathode in the pouch cell was about 1.6 mg cm −2 . Electrolyte was using 1 wt % LiNO 3 additive and 1.0 M LiTFSI in a mixture of DOL and DME with volume ratio of 1:1. The galvanostatic charge/discharge tests were performed with a multi-channel battery test system (Neware CT-3008W, China) in voltage window of 1.7~2.8 V. Cyclic voltammetry (CV) measurement was conducted using a PARSTAT multichannel electrochemical workstation (Princeton Applied Research, USA).
Electrochemical impedance spectrums (EIS) were performed with a frequency range from 100 kHz to 0.1 Hz and AC amplitude of 5 mV. For the in-situ XRD measurement, the cathode was prepared by coating the slurry onto Al foil and the in-situ XRD cell was cycled at 1.7~2.8 V at 0.05 C rate with a Landt CT2001A battery test system.

Li metal anode tests
TiO 2 @VN-Li working electrode was prepared by mixing 90 wt% TiO 2 @VN and 10 wt% PVDF in NMP to form the homogeneous slurry and then coated on Cu foil, which was punched into a disk with a diameter of 12 mm. The lithium foil was used as the counter electrode in CR2032 coin cell and 40 μL above-mentioned electrolyte was added in each cell.
Assembled cells were primarily cycled from 0 to 1 V for 5 cycles at a current density of 50 μA to stabilize the interface.

Li-S full cell tests
Li-S full cells were assembled by using TiO 2 @VN-S as the cathode, TiO 2 @VN-Li as the anode, and the Celgard 2400 separator into a CR2032 coin cell. The sulfur loading was controlled at 1.6 mg cm −2 , while the TiO 2 @VN-Li anode was prepared by pre-plating 4 mAh cm −2 of Li onto the TiO 2 @VN-coated Cu foil. For a higher sulfur loading of 3.6 mg cm −2 , the TiO 2 @VN-Li anode was prepared by pre-plating 9 mAh cm −2 of Li onto TiO 2 @VN-coated Cu foil. The E/S ratio was 15 µL mg −1 S and 10 µL mg −1 S at the sulfur loading of 1.6 and 3.6 mg cm −2 respectively. The contrast sample was assembled using CNT-S cathode and Cu-Li anode without TiO 2 @VN modification. The galvanostatic charge/discharge test was performed with Neware CT-3008W tester at different current densities within a voltage window of 1.7~2.8 V.

Lithium polysulfide adsorption tests
The 5.0 mmol L −1 Li 2 S 6 solution was prepared by dissolving stoichiometric amounts of S and Li 2 S with a molar ratio of 5:1 in a mixed solvent of 1, 2-dioxolane (DOL) and dimethoxymethane (DME) (1:1 by volume) in an argon-filled glove-box. Typically, visualized adsorption tests were carried out by adding 20 mg of TiO 2 @VN, 40 mg of TiO 2 and 10 mg of VN composites (based on approximately the same surface area) into 3 mL of the as-prepared Li 2 S 6 solution respectively. The concentration of the residual lithium polysulfide in the solution was determined by using a UV-vis absorption spectrophotometry (UV-vis, Shimadzu UV 3600).

Symmetric cell tests
TiO 2 @VN and polyvinylidene fluoride (PVDF) were mixed by a weight ratio (9:1) in Nmethyl-pyrrolidone (NMP) solvent, and then coated on the Al/C foil. After drying at 60 °C for 12 h, the electrode was punched into disks with a diameter of 12 mm. The active material loading was controlled at about 1.5 mg cm −2 . Li 2 S 6 symmetric cells were assembled by employing two identical TiO 2 @VN electrodes, Celgard 2400 separator, and 40 μL of Li 2 S 6 electrolyte (0.125 M). The CV tests were performed between −1.0 and 1.0 V by a PARSTAT electrochemical workstation (Princeton Applied Research, USA).

Nucleation and dissolution behavior of Li 2 S studies
To investigate the liquid-solid reaction kinetics, the TiO 2 @VN coated on Al/C foil was assembled into the CR2032 coin cell by paring Li foil as anode. 20 μL Li 2 S 8 catholyte (composed of 0.3 M Li 2 S 8 and 1.0 M LiFSI dissolved in tetraglyme solution) was added into the cathode side and 20 μL tetraglyme solution was added into the anode side. For the nucleation of Li 2 S studies, the cell was galvanostatically discharged to 2.06 V, and then discharged potentiostatically at 2.05 V until the current was lower than 10 −5 A. To analyze the Li 2 S dissolution behavior, fresh cells were first discharged at a current of 0.10 mA to 1.80 V, and subsequently discharged at 0.01 mA to 1.80 V for full transformation of S species into solid Li 2 S. After this discharge, cells were potentiostatically charged at 2.40 V for the dissolution of Li 2 S into LiPS until the charge current was below 10 −2 mA.

Materials characterizations
The structure and phase composition analyze of all materials were tested by X-ray diffraction (XRD) patterns conducted with a Bruker DX-1000 diffractometer with Cu Ka radiation (λ=1.54178 Å, Philips X'pert TROMPD). The density of TiO 2 @VN composites was tested by fully automatic true density analyzer (American Mike automatic true density tester AccuPyc II 1340). Electronic conductivity tests of TiO 2 @VN, TiO 2 and VN composites were measured by four-probe direct current method using ST2253 type digital four-probe tester. The field-  Table S1.
Nitrogen adsorption/desorption measurements were performed on a Kubo-X1000 analyzer (Beijing Builder Electronic Technology Co., Ltd). Thermogravimetric analysis (TGA, METTLER TOLEDO, USA) was carried out at a temperature range of 25-600 °C with a heating rate of 10 °C min −1 in nitrogen atmosphere to analyze the content of sulfur in the composites.

Density functional theory calculation
The structural optimizations and electronic structure calculations are performed based on density functional theory (DFT) as implemented in the Vienna Ab Initio Simulation Package (VASP) code, [S5] based on the projector augmented wave (PAW) method with a cutoff energy of 600 eV. [S6] All of configurations of TiO 2 @VN based materials were fully optimized. [S5] The generalized gradient form (GGA) of the exchange-correlation functional (Perdew-Burke-Ernzerhof 96, PBE) was adopted. [S7, S8] A revised Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol) [S9, S10] was used for the exchange-correlation. PBEsol functional has been introduced to improve the equilibrium properties of solids. [S11] Valencecore interactions were described by projector-augmented-wave (PAW) pseudopotentials. [S12] The Brillouin zone sampling is carried out using the (3×3×1) Monkhorst-Pack grids for surface and Gamma for the structure. [S6] The convergence tolerance of energy is 1×10 −5 eV, maximum force is 0.002 eV Å −1 , and maximum displacement is 0.002 Å. [S6]

+ )
The D Li + can be calculated by the classic Randles-Sevcik equation: where Ip is the peak current (mA), n is the charge transfer number per reaction species (n=2), A is the geometric electron area (1.13 cm 2 here), C is the concentration of lithium ions (10 −3 mol cm 3 here), and v is the scan rate (V s −1 ). According to the above equation, the calculated slope values and D Li + are listed in Table S2.

Electrode porosity
The electrode porosity (ε) is determined from the relation: [S13] where m areal (g cm −2 ) is the mass loading of the electrode except current collector, L(cm) is the electrode thickness, and w (%) and ρ (g cm −3 ) are the mass fraction and real density of every component, respectively.

Calculation of energy density
The cathode-level (based on the cathode) volumetric energy density (E V ) and gravimetric energy density (E G ) of half-cell was calculated according to the following equation: where m is the mass of cathode (mg), Q is the capacity of the cathode (mAh g −1 ), U is the voltage at half specific capacity during discharge (V), d is the thickness of cathode (µm), and S is the area of electrode (cm −122 ).
The electrode-level (based on the cathode and the anode) volumetric energy density (E V ) and gravimetric energy density (E G ) of full-cell was calculated according to the following equation: where m is the mass of cathode and anode (mg), Q is the capacity of the cathode (mAh g −1 ), U is the voltage at half specific capacity during discharge (V), d is the total thickness of cathode and anode (µm), and S is the area of electrode (cm −2 ).
For the TiO 2 @VN-S cathode in half cells with the sulfur loading of 1.6 mg cm −2 at a current density of 0.1 C, the E V and E G (based on the cathode) were calculated as follows: For the TiO 2 @VN-S cathode with the sulfur loading of 4.2 mg cm −2 at a current density of 0.5 mA cm −2 , the E V and E G (based on the cathode) were calculated as follows: For the TiO 2 @VN-S||Cu-Li full cell with the sulfur loading of 3.6 mg cm −2 at a current density of 0.1 C, the electrode-level (based on the cathode + anode) volumetric energy density E V and E G were calculated as follows: Where the mass of cathode is 8.36 mg, and the mass of anode is 5.38 mg (2.63 mg Li and 2.75 mg TiO 2 @VN).  Figure S1 Zeta potentials of the CFs and Ti 4+ @CFs dispersed in water at pH value of 4.0.

Figure S20
Cycling performance of the TiO 2 @VN-S at 1 C.

Figure S21
Digital photographs of a Li-S pouch cell in various bent states to continuously light up a "SCU" device containing 60 red light-emitting diodes (LEDs).

Figure S23
High-resolution XPS spectra of S 2p for the TiO 2 @VN after interacting with Figure S24 Schematic illustration of the role of TiO 2 , VN and TiO 2 @VN in tuning S chemistry.

Figure S26
Cross-section SEM images of (a) TiO 2 @VN-Li and (b) Cu-Li electrodes with pre-plating 4 mAh cm −2 of Li.

Figure S27
Cross-section SEM images of (a) TiO 2 @VN-S at sulfur loading of 3.6 mg cm −2 and (b) TiO 2 @VN-Li electrodes with pre-plating 9 mAh cm −2 of Li.